With recent drivers and recent enough DEs it is usable to a certain extent. There are some known issues document in the release notes of each version. Here’s for 535. TL; DR the major blockers are: (1) Variable Refresh Rate doesn’t work for some cards; (2) GAMMA_LUT is not implemented (no night light) and (3) Nested X11 clients have synchronisation issues that might result in some flickering or dupicate frames, it’s more noticeable on things that refresh slowly, although much better recently. Also (4) NvFBC capture doesn’t work with nested X11 clients which might or might not be important for some people.

I’m using it with Plasma; it’s OK, no major concerns but my setup is pretty basic.